Optical waveguide and manufacturing method thereof

ABSTRACT

An optical waveguide, comprising a cladding around a core, wherein this optical waveguide is configured such that an adhesive layer is interposed between the core and the cladding; as well as a method for manufacturing an optical waveguide comprising a core and a cladding, wherein this method for manufacturing an optical waveguide comprises forming a core film and a first cladding layer with a core groove, embedding the core film into the groove of the first cladding layer, forming an adhesive film, providing a second cladding layer for forming an integrally bonded structure containing the first cladding layer, and bonding the first and second cladding layers together through the agency of the adhesive film. This invention provides an optical waveguide whose transmission loss does not depend on wavelength in the optical communications waveband of 0.6-1.55 μm, wherein this optical waveguide had adequate adhesion strength between cladding layers or the like and possesses high peel strength; and to provide a manufacturing method that allows an optical waveguide devoid of cladding layer cracking or core shifting to be obtained with ease.

FIELD OF THE INVENTION

[0001] The present invention relates to an optical waveguide that can beused as the internal inter-equipment wiring, printed board internalwiring, optoelectronic integrated circuit, or other component of opticalcommunications equipment for transmitting optical signals, and to amethod for manufacturing such a waveguide; and more particularly to anoptical waveguide configured such that the core and cladding of theoptical waveguide are fashioned from an amorphous fluororesin, and to amethod for manufacturing such a waveguide.

BACKGROUND OF THE INVENTION

[0002] With the current development of advanced information exchangedevices, optical fiber networks have gained wide acceptance as the trunklines of communications systems, and high-speed, high-volumetransmissions have already been implemented. Designing inexpensive,easily usable optical components is indispensable for performinghigh-speed, high-volume transmissions on a wider scale through suchoptical fiber networks, and a need exists for creating opticalwaveguides that have adequate optical transmission characteristics andcan accommodate such optical components in densely mounted arrangements.In conventional practice, such optical waveguides have a core and acladding, and inorganic quartz have primarily been contemplated as amaterial for such waveguides because of considerations related to lowoptical transmission loss, the ability to control the refractive indexdifference of the core and the cladding, excellent heat resistance, andthe like. However, using quartz to produce an optical waveguide having acore and a cladding not only involves performing complex steps but alsonecessitates performing high-temperature treatments and createsproduction-related problems.

[0003] By contrast, polymer materials, which are easy to produce and donot require the high-temperature heat treatments commonly needed forquartz or the like, can also be used for optical waveguides. Forexample, optical waveguides whose core is obtained using the organicpolymer material polymethacrylate (PMMA) are widely used because theycan be molded at a low temperature, have excellent workability, are easyto handle, and have other advantages, but an optical waveguide whosecore is obtained using such polymethacrylate loses a substantial amountof light through absorption in the wavelength range used for opticaltransmissions (0.6-1.55 μm; for example, 8.8 μm or greater), making itimpossible to bring optical transmission loss to a sufficiently lowlevel.

[0004] Attempts have therefore been made to reduce the opticaltransmission loss in the aforementioned specific wavelength range ofoptical transmission by designing optical waveguides composed of afluorinated PMMA material exposed to electron beams (JP (Kokai)7-92338), or optical waveguides composed of fluorinated polyimideshaving excellent heat resistance (JP (Kokai) 4-235505). A drawback ofsuch optical waveguides, however, is that light is still lost throughabsorption in the 1.55-μm waveband and that efforts still need to bemade to achieve a sufficient reduction in transmission loss within thisoptical communication band.

[0005] An optical waveguide whose core is obtained using an amorphousfluororesin with low optical transmission loss has therefore beenproposed (JP (Kokai) 4-190292) in order to achieve an adequate reductionin optical transmission loss within the aforementioned opticaltransmission waveband (0.6-1.55 μm), and it was found that an opticalwaveguide with an amorphous fluororesin core does indeed have excellentlight transmission in the 1.55-μm waveband. Such an optical waveguide,however, does not have any cladding and is obtained merely by placingthe core on a silicon wafer. The absence of a cladding causes dust toadhere to the core, and the resulting deposit has a profoundly adverseeffect on optical transmission characteristics and makes the waveguideless suitable for practical use.

[0006] It has been proposed to overcome this shortcoming by employing anoptical waveguide whose core is composed of an amorphous fluororesin,and configuring this optical waveguide such that its core is covered bya cladding (JP (Kokai) 10-227931). This optical waveguide is obtained bya method in which an amorphous fluororesin solution obtained using aspecific fluorocarbon solvent is spin-coated onto a ceramic substrate orsilicon wafer, the coated substrate or wafer is dried to fashion theamorphous fluororesin into a lower cladding layer, this lower claddinglayer is coated with the same amorphous fluororesin solution obtainedusing a specific fluorocarbon solvent, the coated layer is dried to forma core layer, the core layer is coated with the same amorphousfluororesin solution obtained using a specific fluorocarbon solvent, andthe coated layer is dried to form an upper cladding layer. However, theoptical waveguide thus formed develops the following problems when thelower cladding layer, core layer, and upper cladding layer are formedduring manufacturing, and particularly when the aforementioned amorphousfluororesin solution is applied by spin coating to the lower claddinglayer in order to form a core layer or upper cladding layer on the lowercladding layer.

[0007] Specifically, the lower cladding layer is dissolved by thefluorocarbon solvent of the amorphous fluororesin solution used for thecore layer or upper cladding layer between the already formed lowercladding layer of the amorphous fluororesin and the amorphousfluororesin solution subsequently applied as a core layer or uppercladding layer, making it extremely difficult to fabricate adequateoptical waveguides with good accuracy and reproducibility because of thefact that stress release, swelling, and mixing occur; the lower claddinglayer develops cracks; the core shifts away from its prescribedformation position; and the like.

[0008] Another drawback is that when an amorphous fluororesin solutionobtained by dissolving the aforementioned specific fluorocarbon solventis used to form the amorphous fluororesin as a lower cladding layer on aceramic substrate or silicon wafer, the amorphous fluororesin adheresextremely poorly to the ceramic substrate or silicon wafer, and thelower cladding layer tends to peel off from the ceramic substrate orsilicon wafer, yielding inadequate adhesive strength or peel strength.

SUMMARY OF THE INVENTION

[0009] An object of the present invention, which was perfected in viewof the aforementioned problems, is to provide an amorphous fluororesinoptical waveguide that has excellent heat resistance, improved waterresistance, low optical absorption loss, and no dependence oftransmission loss on wavelength in the waveband (0.6-1.55 μm) used foroptical communications, wherein this amorphous fluororesin opticalwaveguide resists peeling and has adequate adhesive strength between thelower cladding layer formed on a substrate (such as a ceramic substrateor silicon wafer) and the substrate itself, or between the lowercladding layer and an upper cladding layer; and to provide a method formanufacturing an amorphous fluororesin optical waveguide with goodreproducibility and high accuracy such that the optical waveguide iseasy to manufacture, the cladding layer is devoid of cracks, and thecore remains in place after being formed at a prescribed location.

[0010] The invention provides an optical waveguide, having a lowercladding layer having grooves formed therein; a core disposed within thegrooves; an adhesive layer disposed over the lower cladding layer andthe core; and an upper cladding layer disposed over the adhesive layer.The lower cladding layer, the core, the adhesive layer, and the uppercladding layer are preferably made of an amorphous fluororesin. Thelower cladding layer is preferably Teflon AF-1600. The core ispreferably Cytop CTL-809S. The adhesive layer is preferably TeflonAF-1600 and a perfluoropolyether fluorine oil. The upper cladding layeris preferably Teflon AF-1600. The invention also provides a substrateadjacent to the lower cladding with another adhesive layer disposedbetween the lower cladding layer and the substrate. The substrate ispreferably a silicon wafer. The other adhesive layer is preferably CytopCTL-809M.

[0011] This invention also provides a method of making an opticalwaveguide by the steps of

[0012] (a) providing a substrate;

[0013] (b) forming a lower adhesive layer over the substrate;

[0014] (c) forming a lower cladding layer over the lower adhesive layer;

[0015] (d) forming grooves in the lower cladding layer;

[0016] (e) forming a core in the grooves;

[0017] (f) forming an upper adhesive layer over the core and the lowercladding layer; and

[0018] (g) forming an upper cladding layer over the upper adhesivelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a cross-sectional view depicting an optical waveguideaccording to one embodiment of present invention.

[0020] FIGS. 2(1) to 2(6) are schematic drawings depicting theproduction sequence of the optical waveguide shown in FIG. 1.

[0021] FIGS. 3(1) to 3(3) are schematic drawings depicting theproduction sequence of an optical waveguide according to anotherembodiment of the present invention.

[0022] FIGS. 4(1) to 4(3) are schematic drawings depicting theproduction sequence of an optical waveguide according to anotherembodiment of the present invention.

[0023]FIG. 5 is a diagram illustrating a laminated optical waveguideaccording to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The stated object is attained by the inventive optical waveguideand manufacturing method. Specifically, the present invention primarilyresides in an optical waveguide that comprises a cladding around a core,wherein this optical waveguide is configured such that an adhesive layeris interposed between the core and the cladding, and also in a methodfor manufacturing an optical waveguide by forming a first cladding layerwith a core groove, providing a core film, embedding the core film intothe groove of the first cladding layer, forming an adhesive film,providing a second cladding layer for forming an integrally bondedstructure containing the first cladding layer, and bonding the first andsecond cladding layers together through the agency of the adhesive film.

EXAMPLES

[0025] The present invention will now be described through workingexamples with reference to the accompanying drawings.

[0026] In FIG. 1 depicts an amorphous fluororesin optical waveguide 1fashioned in accordance with the present invention. The opticalwaveguide 1 may, for example, comprise a substrate 2 in the form of asilicon wafer. An amorphous fluororesin lower cladding layer 4 (firstcladding layer) composed of Teflon AF1600 (manufactured by Du Pont) isformed on the substrate 2 via an interposed lower adhesive layer 3(Cytop CTL-809M, manufactured by Asahi Glass), which is based on theamorphous fluororesin described in detail below. An amorphousfluororesin core layer 5 composed of Cytop (CTL-809S, manufactured byAsahi Glass) is formed on the lower cladding layer 4, and an amorphousfluororesin upper cladding layer 7 (second cladding layer) composed ofTeflon AF1600 (manufactured by Du Pont) is formed in the same manner onthe core layer 5 via an interposed amorphous fluororesin upper adhesivelayer 6 (Teflon AF160 containing fluorine ions, such as Krytox GPL-107from Du Pont), which is based on the amorphous fluororesin described indetail below.

[0027] A method for manufacturing the inventive amorphous fluororesinoptical waveguide thus configured will now be described with referenceto FIG. 2.

[0028] To manufacture an amorphous fluororesin optical waveguide 1 inaccordance with the present invention, a 9-wt % amorphous fluororesinsolution (Cytop CTL-809M from Asahi Glass) is applied, for example, byspin coating to a substrate 2 shaped, for example, as a silicon waferwith a diameter of about 10 cm and a thickness of about 0.5 mm, as shownin FIG. 2; and the coated substrate is heated to remove the solvent,yielding a highly adhesive lower adhesive layer 3 with a thickness ofabout 0.1 μm on the substrate 2 (see FIG. 2(1)). In this case, thearrangement in which an amorphous fluororesin solution is applied byspin coating to the substrate 2 to form a lower adhesive layer 3 can bereplaced with an arrangement in which a film shaped as a very thin diskis fabricated in advance as an adhesive layer in a separate step, andthis film is mounted on the substrate 2 and used as an adhesive layer.

[0029] A round film with a diameter of about 10 cm and a thickness ofabout 80 μm (fabricated in advance by coating, casting, or the like in aseparate step) is subsequently mounted on the lower adhesive layer 3 asa lower cladding layer 4. The lower cladding layer 4 is sufficientlythick to function as the lower cladding layer 4 of an optical waveguidewhen provided with a groove for the below-described core layer (see FIG.2(2)). The film 4 used herein can be obtained by a method in which anamorphous fluororesin solution obtained by dissolving an amorphousfluororesin (Teflon AF-1600 from Du Pont; refractive index: 1.31) in afluorocarbon solvent (Fluorinert FC-43 from 3M) in a concentration of 8wt % is applied by spin coating in a prescribed thickness (for example,1 mm) to a separately prepared film-manufacturing plate such as asilicon wafer (not shown); the film-manufacturing plate (not shown)coated with the amorphous fluororesin solution is introduced into aclean oven (not shown) and heated (under heating conditionscorresponding to stepped heating in which the plate is kept at about100° C. for 1.5 hours, the temperature is then raised from 100° C. to160° C. over a period of 8 hours, the plate is kept at 160° C. for 3hours, the system is further heated at a constant rate from 160° C. to300° C. over a period of 3 hours, and the plate is kept at 300° C. for 1hour); the system is then allowed to cool naturally to room temperature;the fluorocarbon solvent is removed; the plate is immersed in an IPA(isopropyl alcohol) solution; and the coating is peeled off from thefilm-manufacturing plate (not shown). Residual stress can thus beremoved from the resulting film. In preferred practice, the amorphousfluororesin solution is applied while rims for preventing the solutionfrom flowing are provided around the film-manufacturing plate (notshown) in order to prevent the amorphous fluororesin solution fromflowing during the application of the amorphous fluororesin solution tothe film-manufacturing plate (not shown).

[0030] The lower cladding layer 4 in the form of a round film is thusmounted on the lower adhesive layer 3, and the lower cladding layer 4 issecurely bonded to the substrate 2 via the lower adhesive layer 3 by aheat fusion technique in which the system is gradually heated while keptat a reduced pressure (for example, a pressure of 0.38 kg/cm² isapplied) with the aid of a vacuum press, a temperature of 206° C. ismaintained for 1 minute, and the system is allowed to cool naturally toroom temperature. In the process, the lower cladding layer 4 shaped as afilm is mounted on and bonded to the lower adhesive layer 3, making itpossible to prevent stress release, swelling, mixing, or contaminationfrom occurring between the lower adhesive layer 3 and the lower claddinglayer 4 in the manner observed when a conventional amorphous fluororesinsolution is used.

[0031] The surface of the lower cladding layer 4 bonded to the substrate2 in this manner is subjected to reactive ion etching (hereinafterreferred to as “RIE”), and the surface of the lower cladding layer 4 isthus modified. The etching is carried out such that oxygen is fed to theRIE apparatus at a rate of 200 sqm, the pressure inside the RIEapparatus is kept at 45 pascal, power is supplied to the RIE apparatusin an amount of 200 W, and the process is continued for 30 seconds.

[0032] A resist 8 (positive resist OFPR-8600 from Tokyo Ohka) is appliedby speed coating to the surface of the lower cladding layer 4 modifiedby RIE, and the coated layer is pre-baked for 40 minutes at 80° C. Anegative mask pattern prepared in advance is subsequently applied to thesurface of the lower cladding layer 4, and the layer is exposed using anexposure apparatus, developed with a developing solution, thoroughlywashed, dried, and post-baked for 40 minutes at 120° C., forming apattern on the resist 8 (see FIG. 2(3)).

[0033] The lower cladding layer 4 is then dry-etched in accordance withthe pattern (formed by the resist 8 on the lower cladding layer 4) to aprescribed width (for example, 50 μm) and depth (for example, 8 μm) withthe aid of an RIE apparatus, yielding a groove 9 for the aforementionedcore layer. The etching is carried out such that oxygen is fed to theRIE apparatus at a rate of 10 sqm, carbon tetrafluoride is fed at a rateof 20 sqm, argon is fed at a rate of 90 sqm, the pressure inside the RIEapparatus is kept at 45 pascal, power is supplied to the RIE apparatusin an amount of 200 W, and the process is continued for 20 minutes. Alower cladding layer 4 whose surface is provided with a patterned groove9 is obtained when the resist 8 is peeled off after the substrate 2 inwhich the groove 9 is formed in the lower cladding layer 4 in thismanner is dipped in a resist release solution heated to 100° C. (seeFIG. 2(4)).

[0034] An amorphous fluororesin film 10 designed for the core, shaped asa thin Cytop (CTL-809S from Asahi Glass) disk with a diameter of about10 cm and a thickness of 6 μm, and prepared in advance by hot pressing,film casting, or another special technique is subsequently placed overthe lower cladding layer 4 provided with the groove 9, and the core film10 is melted and allowed to fill the entire groove 9 when, for example,a pressure of 0.38 kg/cm² is applied for 2.5 hours at 120° C. with theaid of a vacuum press in a reduced-pressure environment through theagency of a flat pressure member made of metal (not shown) and placed onthe layer. The assembly is then cooled to room temperature, and theexcess core film 10 remaining on the surface of the lower cladding layer4 is removed using an RIE apparatus. The process is performed under thesame RIE conditions as those established when the groove 9 is formed inthe lower cladding layer 4, and the etching time is adjusted dependingon the excess core film. A patterned core 5 is thus formed on the lowercladding layer 4 (FIG. 2(5)).

[0035] The aforementioned core film 10 can be obtained in the samemanner as the film for the lower cladding layer 4 by a method in whichan amorphous fluororesin (Cytop CTL-809S from Asahi Glass; refractiveindex: 1.34) is applied by spin coating in a prescribed thickness to aseparately prepared film-manufacturing plate such as a silicon wafer(not shown); the film-manufacturing plate (not shown) coated with theamorphous fluororesin solution is introduced into a clean oven (notshown) and heated (under heating conditions corresponding to steppedheating in which the plate is kept at 100° C. for 0.5 hour, thetemperature is then raised from 100° C. to 300° C. over a period of 45minutes, and the plate is kept at 300° C. for 30 minutes), the system isthen allowed to cool naturally to room temperature; the solvent isremoved; the plate is immersed in an IPA (isopropyl alcohol) solution;and the coating is peeled off from the film-manufacturing plate (notshown). Residual stress can thus be removed from the resulting film. Inpreferred practice, the amorphous fluororesin solution is applied whilerims for preventing the solution from flowing are provided around thefilm-manufacturing plate (not shown) in order to prevent the amorphousfluororesin solution from flowing during the application of theamorphous fluororesin solution to the film-manufacturing plate (notshown).

[0036] A film shaped as a thin disk with a diameter of about 10 cm and athickness of about 6 μm and prepared in advance by hot pressing, filmcasting, or another special technique is subsequently placed as an upperadhesive layer 6 on the lower cladding layer 4 provided with the core 5.The film is obtained by a method in which an amorphous fluororesinsolution is prepared by adding a perfluoropolyether fluorine oil such asKrytox (Krytox GPL-107 from Du Pont) in an amount of 8.5 wt % to an 8-wt% solution resulting from the dissolution of an amorphous fluororesin(Teflon AF-1600 from Du Pont, refractive index: 1.31) in a fluorocarbonsolvent (Fluorinert FC-43 from 3M), the resulting amorphous fluororesinsolution containing Krytox GPL-107 is applied by spin coating to afilm-manufacturing plate in the form of a silicon wafer with a diameterof about 10 cm, the solvent is dried off, the coated plate is immersedin an IPA (isopropyl alcohol) solution, and the coating is peeled offfrom the film-manufacturing plate (not shown). Adding theperfluoropolyether fluorine oil allows the glass transition point of theamorphous fluororesin constituting the film to be reduced by about 20degrees.

[0037] An amorphous fluororesin film (diameter: about 10 cm; thickness:80 μm) prepared in advance in a separate step under the same conditionsas those established during the formation of the film for the lowercladding layer 4 is subsequently placed as an upper cladding layer 7 onthe upper adhesive layer 6. The upper cladding layer 7 in the form of around film is thus mounted on the upper adhesive layer 6, and the uppercladding layer 7 is securely and easily bonded to the lower claddinglayer 4 via the upper adhesive layer 6 by a fusion technique in whichthe system is gradually heated from room temperature while kept at areduced pressure (for example, a pressure of 0.38 kg/cm² is applied)with the aid of a vacuum press, the conditions are kept unchanged oncethe temperature reaches 166° C., and the system is then allowed to coolnaturally to room temperature. This process makes it possible to preventswelling, mixing, or contamination from affecting the lower claddinglayer in the manner observed when a solution is applied in order to forman upper cladding layer on a conventional lower cladding layer.

[0038] The bonding process is performed using an upper adhesive layer 6,which is obtained by a method in which the glass transition point of theamorphous fluororesin constituting the upper adhesive layer 6 is keptabout 20 degrees below that of the lower cladding layer 4 and uppercladding layer 7 by the addition of fluorine oil to the upper adhesivelayer 6 in the form of a film in the above-described manner, and theupper cladding layer 7 is bonded to the lower cladding layer 4 throughthe agency of the upper adhesive layer 6 to which the fluorine oil hasbeen added, dispensing with the need to maintain a significantly highertemperature and making it possible to easily bond the upper claddinglayer 7 and the lower cladding layer 4 when the upper cladding layer 7is bonded to the lower cladding layer 4. Consequently, the uppercladding layer 7 and the lower cladding layer 4 can be bonded with easewhile the core 5 is prevented from being melted or otherwise adverselyaffected by a temperature increase, taking into account that, incomparison with the lower cladding layer 4 or upper cladding layer 7,the core 5 is melted more easily by the increased temperatureestablished during bonding. As a result, a highly accurate, completelyembedded amorphous fluororesin optical waveguide can be obtained whilethe lower cladding layer 4 is prevented from cracking, swelling, ormixing, and the core 5 of the groove 9 in the lower cladding layer 4 isprevented from shifting because the core 5 (which is embedded in thegroove 9 formed at a prescribed location of the lower cladding layer 4)is held inside the groove 9 even in the case of partial melting, forexample (FIG. 2(6)).

[0039] Thus, using an upper adhesive layer 6 to which fluorine oil hasbeen added allows the upper cladding layer 7 to be easily bonded to thelower cladding layer 4 while the lower cladding layer 4 is preventedfrom swelling, mixing, core shifting, or the like.

[0040] In comparison with the material constituting the core 5, thematerial constituting the upper cladding layer 7 and the lower claddinglayer 4 is more difficult to melt and has greater heat resistanceagainst the temperature increases effected during bonding when the uppercladding layer 7 is heated and bonded to the lower cladding layer 4; andthe groove 9 of the lower cladding layer 4 preserves the shape of thecore 5 during such heating and bonding when, for example, the core 5 ispartially melted.

[0041] The optical waveguide thus produced was cut with a dicingapparatus to obtain an optical-waveguide end face with a width of 50 μm,a thickness of 8 μm, and a length of 40 mm; white light was shone;outgoing radiation was observed under an optical microscope; and it wasfound that light was confined inside the core 5. Transmission loss(including continuous loss) was also measured by a cutback technique,and it was found that the resulting optical waveguide 1 had excellentoptical characteristics and a negligible dependence on wavelength (0.043dB/cm at a wavelength of 850 nm, 0.046 dB/cm at a wavelength of 1300 nm,and 1.00 dB/cm at a wavelength of 1500 nm).

[0042] Although the above working example was described with referenceto a structure in which the lower cladding layer was directly etchedwhen a core groove was formed in the lower cladding layer, the presentinvention can also be used to manufacture an amorphous fluororesinoptical waveguide by forming a core groove in the lower cladding layerwith the aid of a casting system.

[0043] Specifically, the amorphous fluororesin optical waveguide of thepresent invention can be produced by a method in which, for example, asilicon wafer having specially patterned ribs 3 a is used as a mold 31(FIG. 3(1)) to form a lower cladding layer 4 (as shown, for example, inFIG. 3); rims are placed around the mold 31; the amorphous fluororesinsolution used for the formation of the lower cladding layer 4 in theabove-described working example is poured into the mold 31; the solutionin the mold is heated and dried; the lower cladding layer 4 is formedsuch that the ribs 3 a of the mold 31 form the core groove 9 of thelower cladding layer 4; and the core groove 9 is transferred from themold 31 to the lower cladding layer 4. The reason rims are providedaround the aforementioned mold is to prevent the amorphous fluororesinsolution from flowing when the amorphous fluororesin solution is pouredinto the mold 31, and to form a lower cladding layer 4 of prescribedthickness. With the lower cladding layer 4, the technical scope of thepresent invention also includes arrangements in which a pre-moldedamorphous fluororesin film is placed on a silicon-wafer mold 31 andpressed under heating, yielding a lower cladding layer 4.

[0044] An adhesive layer 3 identical to the lower adhesive layer 3 ofthe above working example is used to place the substrate 2 (siliconwafer) on the opposite side from the lower cladding layer 4 (which isprovided with the core groove 9 in the above-described manner), thesystem is gradually heated from room temperature while kept at a reducedpressure (for example, a pressure of 0.38 kg/cm² is applied) with theaid of a vacuum press, a temperature of 206° C. is maintained for 1minute, the system is then allowed to cool naturally to roomtemperature, and the lower cladding layer 4 is bonded to the substrate 2(FIG. 3(2)). The lower cladding layer 4 bonded to the substrate 2 bymeans of the adhesive layer 3 is then peeled off from the silicon-wafermold 31, making it possible to obtain a lower cladding layer 4 having apatterned groove 9 (FIG. 3(3)). The subsequent procedure is similar tothe one performed in the above-described working example. In otherwords, the core 5 is formed by embedding the amorphous fluororesin film10 for the core in the groove 9 of the resulting lower cladding layer 4with the aid of a heat press or the like, excess core film is thenremoved, and an upper cladding layer 7 is then provided via aninterlying upper adhesive layer 6, yielding an optical waveguidecomposed of an amorphous fluororesin. An optical waveguide composed ofan amorphous fluororesin can thus be formed using a mold such as asilicon wafer.

[0045] Flexible amorphous fluororesin optical waveguides can be producedby the present invention in addition to the waveguides described withreference to the above working examples.

[0046] Specifically, as shown in FIG. 4, a flexible amorphousfluororesin optical waveguide 41 can be obtained in accordance with thepresent invention by a method in which aluminum is vapor-deposited in athickness of about 50 nm by a vacuum vapor deposition apparatus on oneside of the substrate 2 (silicon wafer) used in the above-describedworking example, yielding a dummy layer 42 over the portion formed bythe vapor deposition of aluminum (FIG. 4(1)); and an optical waveguide41 composed of an amorphous fluororesin is formed on the dummy layer 42in the same manner as in the case of the substrate 2 pertaining to theabove working example. Specifically, a lower adhesive layer 3 is formedon the dummy layer 42, a lower cladding layer 4 is formed thereon, apatterned groove 9 is formed in the lower cladding layer 4, theamorphous fluororesin film 10 for the core is then embedded in thegroove 9 by hot pressing or the like to form the core 5, excess corefilm is removed, and an upper cladding layer 7 is provided through theagency of an upper adhesive layer 6 (FIG. 4(2)).

[0047] The substrate 2 provided with the amorphous fluororesin opticalwaveguide 41 is then immersed in an aluminum release solution, whereuponthe vapor-deposited aluminum of the dummy layer 42 is removed, making itpossible to peel off the rigid substrate 2 bonded to the dummy layer ofthe optical waveguide. In other words, the portion devoid of thesubstrate 2 can be provided with a flexible amorphous fluororesinoptical waveguide 41.

[0048] In the process, the dummy layer is configured such that speciallysized rigid portions 2 a are left behind at prescribed positions (forexample, at the two end positions) on the substrate 2 with the amorphousfluororesin optical waveguide 41, yielding a flexible optical waveguide41 in which the specially sized rigid portions of the substrate 2 a aredisposed at the two ends, and the portion of the rigid substrate 2 acorresponding to the central portion is removed (FIG. 4(3)). Theportions of the rigid substrate 2 a can adequately preserve the left andright positions of the optical waveguide at the two ends of the opticalwaveguide 41, and the system can be accurately positioned with minimumeffort by sandwiching these portions of the substrate 2 a in aconnector, making the system easier to assemble. Disposing theseportions of the substrate 2 a slightly further inward from the two endportions of the optical waveguide 41 allows the portions of the opticalwaveguide at the two ends to be displaced with greater ease, making itpossible to place the core in the exact center (that is, to position thecore with greater accuracy) by maintaining the desired thicknessaccuracy of the optical waveguide and the molding accuracy of theconnector without any relation to the dimensional error related to thethickness of a rigid portion such as that of a silicon wafer.

[0049] Another feature of the inventive amorphous fluororesin opticalwaveguide is that because the core and the cladding are formed from anamorphous fluororesin film, the optical waveguide can be optionallyfashioned as a laminated optical waveguide obtained by stackingamorphous fluororesin layers. Specifically, the laminated opticalwaveguide can be formed by a method in which a second core 5 a is formedon the surface of the upper cladding layer 7 constituting the opticalwaveguide of the above working example in the same manner as the core 5formed on the aforementioned lower cladding layer 4, as shown in FIG. 5.The same amorphous fluororesin film as that used in the working exampleabove is then placed as a laminated upper cladding layer 7 a on thesecond core 5 a via an interposed film-like upper adhesive layer (notshown), and the laminated upper cladding layer 7 a and the uppercladding layer 7 are integrally bonded together. V-shaped cut surfaceshaving prescribed angles to allow optical signals to reach the core arethen formed on the upper and lower surfaces of the laminated opticalwaveguide in order to ensure that the optical signals can be transmittedbetween the first core 5 and second core 5 a of the laminated opticalwaveguide.

[0050] The laminated optical waveguide has cut surfaces 51 and 52, whichmake an angle of about 45 degrees with the longitudinal direction of thecore, and an optical signal 100 incident on the first core 5 isreflected at 90 degrees by the cut surface 52, admitted into the secondcore 5 a, reflected for a second time by the cut surface 51 of thesecond core 5 a, and transmitted by the second core 5 a. The laminatedoptical waveguide pertaining to the present invention can thus transmitlight in the thickness direction as well.

[0051] The optical waveguide laminate of the present invention iscomposed of a resin, making it easier to form the light-reflecting cutsurfaces.

[0052] It follows from the above description that because the opticalwaveguide and manufacturing method of the present invention involve theuse of an amorphous fluororesin, it is possible to provide an opticalwaveguide that has excellent heat resistance, improved water resistance,low optical absorption loss and minimal wavelength dependence within theoptical communications waveband, high peeling resistance, and adequateadhesive strength between the lower cladding layer on a substrate (suchas a ceramic substrate or a silicon wafer) and the substrate itself, orbetween lower and upper cladding layers. The present invention alsoallows an adequate optical waveguide to be fabricated with high accuracyand improved reproducibility while facilitating production andpreventing situations in which swelling or mixing is brought about bythe application of an amorphous fluororesin solution in the mannerobserved in the past, the cladding layers develop cracks as a result ofstress release, or the core shifts away from its designated formationposition.

What is claimed is:
 1. An optical waveguide, comprising: (a) a lowercladding layer having grooves formed therein; (b) a core disposed withinsaid grooves; (c) an adhesive layer disposed over said lower claddinglayer and said core; and (d) an upper cladding layer disposed over saidadhesive layer.
 2. An optical waveguide as defined in claim 1 whereinsaid lower cladding layer, said core, said adhesive layer, and saidupper cladding layer comprise an amorphous fluororesin.
 3. An opticalwaveguide as defined in claim 2 wherein said lower cladding layercomprises Teflon AF-1600.
 4. An optical waveguide as defined in claim 2wherein said core comprises Cytop CTL-809S.
 5. An optical waveguide asdefined in claim 2 wherein said adhesive layer comprises Teflon AF-1600and a perfluoropolyether fluorine oil.
 6. An optical waveguide asdefined in claim 2 wherein said upper cladding layer comprises TeflonAF-1600.
 7. An optical waveguide as defined in claim 1 furthercomprising a substrate adjacent to said lower cladding with anotheradhesive layer disposed between said lower cladding layer and saidsubstrate.
 8. An optical waveguide as defined in claim 7 wherein saidsubstrate comprises a silicon wafer.
 9. An optical waveguide as definedin claim 7 wherein said another adhesive layer comprises Cytop CTL-809M.10. An optical waveguide comprising: (a) a silicon wafer substrate; (b)a lower adhesive layer comprising amorphous fluororesin disposed oversaid substrate; (c) a lower cladding layer comprising amorphousfluororesin and having grooves formed therein disposed over said loweradhesive layer; (d) a core comprising amorphous fluororesin disposed insaid grooves; (e) an upper adhesive layer comprising amorphousfluororesin and a perfluoropolyether fluorine oil disposed over saidcore and said lower cladding layer; and (f) an upper cladding layercomprising amorphous fluororesin disposed over said upper adhesivelayer.
 11. A method of making an optical waveguide comprising: (a)providing a substrate; (b) forming a lower adhesive layer over saidsubstrate; (c) forming a lower cladding layer over said lower adhesivelayer; (d) forming grooves in said lower cladding layer; (e) forming acore in said grooves; (f) forming an upper adhesive layer over said coreand said lower cladding layer; (g) forming an upper cladding layer oversaid upper adhesive layer.